2.1. Hematopoietic Stem and Progenitor Cells
The morphologically recognizable and functionally capable cells circulating in blood include erythrocytes, neutrophilic, eosinophilic, and basophilic granulocytes, B-, T-, nonB-, non T-lymphocytes, and platelets. These mature cells derive from and are replaced, on demand, by morphologically recognizable dividing precursor cells for the respective lineages such as erythroblasts for the erythrocyte series, myeloblasts, promyelocytes and myelocytes for the granulocyte series, and megakaryocytes for the platelets. The precursor cells derive from more primitive cells that can simplistically be divided into two major subgroups: stem cells and progenitor cells (for review, see Broxmeyer, H. E., 1983, "Colony Assays of Hematopoietic Progenitor Cells and Correlations to Clinical Situations," CRC Critical Reviews in Oncology/Hematology 1(3):227-257). The definitions of stem and progenitor cells are operational and depend on functional, rather than on morphological, criteria. Stem cells have extensive self-renewal or self-maintenance capacity (Lajtha, L. G., 1979, Differentiation 14:23), a necessity since absence or depletion of these cells could result in the complete depletion of one or more cell lineages, events that would lead within a short time to disease and death. Some of the stem cells differentiate upon need, but some stem cells or their daughter cells produce other stem cells to maintain the precious pool of these cells. Thus, in addition to maintaining their own kind, pluripotential stem cells are capable of differentiation into several sublines of progenitor cells with more limited self-renewal capacity or no self-renewal capacity. These progenitor cells ultimately give rise to the morphologically recognizable precursor cells. The progenitor cells are capable of proliferating and differentiating along one, or more than one, of the myeloid differentiation pathways (Lajtha, L.G. (Rapporteur), 1979, Blood Cells 5:447).
Stem and progenitor cells make up a very small percentage of the nucleated cells in the bone marrow, spleen, and blood. About ten times fewer of these cells are present in the spleen relative to the bone marrow, with even less present in the adult blood. As an example, approximately one in one thousand nucleated bone marrow cells is a progenitor cell; stem cells occur at a lower frequency. These progenitor and stem cells have been detected and assayed for by placing dispersed suspensions of these cells into mice, and noting those cells that seeded to an organ such as the spleen and which found the environment conducive to proliferation and differentiation. These cells have also been quantified by immobilizing the cells outside of the body in culture plates (in vitro) in a semi-solid support medium such as agar, methylcellulose, or plasma clot in the presence of culture medium and certain defined biomolecules or cell populations which produce and release these molecules. Under the appropriate growth conditions, the stem or progenitor cells will go through a catenated sequence of proliferation and differentiation yielding mature end stage progeny, which thus allows the determination of the cell type giving rise to the colony. If the colony contains granulocytes, macrophages, erythrocytes, and megakaryocytes (the precursors to platelets), then the cell giving rise to them would have been a pluripotential cell. To determine if these cells have self-renewal capacities, or stemness, and can thus produce more of their own kind, cells from these colonies can be replated in vivo or in vitro. Those colonies, which upon replating into secondary culture plates, give rise to more colonies containing cells of multilineages, would have contained cells with some degree of stemness. The stem cell and progenitor cell compartments are themselves heterogeneous with varying degrees of self-renewal or proliferative capacities. A model of the stem cell compartment has been proposed based on the functional capacities of the cell (Hellman, S., et al., 1983, J. Clin. Oncol. 1:227-284). Self-renewal would appear to be greater in those stem cells with the shortest history of cell division, and this self-renewal would become progressively more limited with subsequent division of the cells.
A human hematopoietic colony-forming cell with the ability to generate progenitors for secondary colonies has been identified in human umbilical cord blood (Nakahata, T. and Ogawa, M., 1982, J. Clin. Invest. 70:1324-1328). In addition, hematopoietic stem cells have been demonstrated in human umbilical cord blood, by colony formation, to occur at a much higher level than that found in the adult (Prindull, G., et al., 1978, Acta Paediatr. Scand. 67:413-416; Knudtzon, S., 1974, Blood 43(3):357-361). The presence of circulating hematopoietic progenitor cells in human fetal blood (Linch, (Fauser, A. A. and Messner, H. A., 1978, Blood 52(6):1243-1248) has also been shown. Human fetal and neonatal blood has been reported to contain megakaryocyte and burst erythroblast progenitors (Vainchenker, W., et al., 1979, Blood Cells 5:15-42), with increased numbers of erythroid progenitors in human cord blood or fetal liver relative to adult blood (Hassan, M. W., et al., 1979, Br. J. Haematol. 41:477-484; Tchernia, G., et al., 1981, J. Lab. Clin. Med. 97(3):322-331). Studies have suggested some differences between cord blood and bone marrow cells in the characteristics of CFU-GM (colony forming unit-granulocyte, macrophage) which express surface Ia antigens (Koizumi, S., et al., 1982, Blood 60(4):1046-1049).